1. Field of the Invention
The present invention relates to a shift control system in a belt-type continuously variable transmission.
2. Description of the Prior Art
Conventionally, as a continuously variable transmission for a vehicle, there is, for example, a belt-type continuously variable transmission with a V-belt. Generally, in the continuously variable transmission, a hydraulic pressure (hereinafter referred to as a primary pulley pressure) which is obtained by regulating a line pressure as an original pressure with a shift control valve is supplied to a cylinder chamber of a primary pulley, and a gear ratio of the vehicle such as an automobile is continuously controlled by increasing and reducing the primary pulley pressure with the shift control valve to change a groove width of the primary pulley and changing a radius ratio between the primary pulley and a secondary pulley. This shift control valve is connected to the vicinity of a middle of a shift link constituting a mechanical feedback mechanism, and a movable flange of the primary pulley and a step motor that is a shift actuator are respectively connected to both ends of the shift link.
When the vehicle in which such a continuously variable transmission is mounted is stopped after it is rapidly decelerated, the vehicle may be stopped before a pulley ratio is returned to a lowest value Lo (low speed side). If the radius ratio between the primary pulley and the secondary pulley is rapidly changed when the pulley is rotated at a low speed, slippage may occur in the V-belt because of the construction of the continuously variable transmission. Therefore, in case the vehicle is restarted after it is stopped from the rapid deceleration as described above, and difference between a target gear ratio (lowest value Lo) at the time of start of the vehicle and an actual gear ratio is large, shift speed is lowered for prevention of slippage of the V-belt. An example of this continuously variable transmission is disclosed in Japanese Patent laid-open patent publication No. 9-203459.
In this conventional continuously variable transmission, in case re-acceleration is performed without stopping the vehicle after the rapid deceleration to the low speed, the vehicle is brought into acceleration before the pulley ratio is returned to the lowest value Lo (low speed side), and a target gear ratio lowest value Lo is commanded to the pulley so that a quick acceleration can be realized. Thus, a problem is involved in that slippage occurs in the V-belt since the actual gear ratio that is not returned to the lowest value Lo at the time of rapid deceleration is forced to the lowest value Lo that is the target gear ratio.